Excess-voltage protective device



United States Patent Office 2,883,573 EXCESS-VOLTAGE PROTECTIVE DEVICERalph R. Pittman, Little Rock, Ark. Application February 14, 1958,Serial No. 715,256 5 Claims. (Cl. 313-231) This invention relatesgenerally to lightning protective devices adapted for use with energizedelectrical circuits for the purpose of limiting the voltage rise of anassociated conductor by discharging the abnormal or superimposed energywhich may result from lightning strokes.

At this time there are in general use in connection with electricaldistribution systems only two types of excessvoltage protective devices,sometimes called lightning arresters.

The first type is usually referred to as a valve type arrester, whichconsists of one or more spark gaps in series with a current-limitingresistance, the resistance material generally used being siliconcarbide, this material having a resistance which varies exponentiallywith applied voltage. This device has good ability for limiting thepower-follow current which tends to flow from the associated energizedconductor to ground when the device sparks over in response to apredetermined voltage, but has serious practical limitations as tosurge-current carrying capacity and further, the voltage drop across theresistance material is of course impressed upon the associated apparatusthe device is intended to protect.

The second type is commonly called an expulsion type alrrester, which isvirtually the converse of the valve type, having relatively poorpower-follow characteristics but practically unlimited surge-currentability with substantially no accompanying voltage drop impressed uponthe apparatus protected. The desirability of the provision of anexcess-voltage protective device having the surge-current carryingcapacity inherent in the expulsion device along with the property ofrestricting the power-follow current to an insignificant amount is quiteapparent, and the present invention relates specifically to such adevice.

Many forms of lightning protective devices have been used, usuallyclassified as expulsion arresters, which embody materials ofconstruction similar to those found to be acceptable for use in thepresent invention.

Usually such devices depend for their operation upon (1) theestablishment of a follow-current are following the precursory dischargespark and (2) the evolution from the adjacent insulating material ofcopious quantities of arc-extinguishing gases to provide a relativelycool and de-ionizing environment for quenching the hot and there foreconducting arc-sustaining gases to effect are extinguishment andinterruption of the follow-current. Such a procedure necessarilyconsumes the materials of which the arc chamber is constructed, and theuseful life of the device is a function of both the number of operationsand the current available for contributing to the intensity of' the arc.

Arc chambers of the devices mentioned generally consist of a fillermember partially filling the hollow of an associated tube, the materialof the filler and tube being formed of some gas-evolving material, suchas hard fiber. Convention-ally-used fillers have been either smooth-sur-2,883,573 Patented Apr. 21, 1959 faced cylindrical members orcylindrical members provided with helical or circumferential grooves.

The undesirability of constructions which permit the flow ofpower-follow current has long been recognized, and some attempts havebeen made to solve this problem by means of some sort of a filler-tubeconstruction having the ability to discharge the transient surgecurrents without initiating a dynamoelectric follow-current arc. Theseattempts are generally exemplified by a filler-tube arrangement in whicha smooth cylindrical plug-type filler is driven tightly into the tube,depending upon the surge voltage to cause a discharge spark to passthrough an extremely restricted path between the inner surface of thetube and the outer surface of the filler. There being no compensatoryadjusting mechanism in such a construction the effectiveness of itsoperation is dependent upon a definite and critical precision of fit ofthe filler plug in the tube.

Since known insulating materials having suitable properties to withstandthe mechanical stresses and temperatures attending operation andparticularly that material most suitable, namely-vulcanized fiberaresubject to important changes in dimensions with changing humidity andtemperature, the vital precise clearance between the members of thearcing chamber is more apt to be absent than not. Slightly too muchclearance results in serious grooving by the follow-current areinitiated by a discharge spark, because this construction is a singledischarge path device, and in the absence of any alternate path, noforces are brought into action to move or spread the discharge sparkfrom the initial path. On the other hand, ifadjacent surfaces of thefiller and tube are pressed together so that all of the air is squeezedout of the space therebetween, then no air gap is available for thepassage of the discharge spark, and the device will fail by externalfiashover, or alternately be protected by failure of the associatedapparatus it is intended to protect.

The principal object of the present invention is the provision of anexcess-voltage protective device, embodying a pair of separate paths,which is capable of (a) discharging a widely varying range of transientsurgecurrents, such surge-currents lasting only for a very smallfraction of a cycle of a cycle power system and (b) restrain-ing thefiow of power-follow current, which in conventional devices continues toflow from the 60-cycle power line through the path initiated by thedischarge spark, to such an extent that the operation of the devicecauses no visible erosion of the insulating members bounding thedischarge path, or indication of followcurrent or 60 cycle voltagedisturbance on the trace when the electrical operation of the device isrecorded by a magnetic oscillograph.

Another object of the invention resides in the provision of means forcompensating for dimensional changes of the insulating members boundingthe discharge path, so that the ability of device to discharge surgeswithout accepting follow-current as above described is unaifected byeither weather conditions or discharge sparks.

' Another object of the invention resides in the provision of a novelfiller construction which renders the device capable of discharginghigh-current surge sparks without eroding a groove at the path followedby the surge current, such a filler being provided with a helical furrowforming a relatively narrow channel extending from one end of the fillerto the other, with an intervening land between the convolutions of thenarrow channel which is substantially wider than the channel.

Another object is an improved plural-part filler construction dividednear its midpoint along abutting oblique end surfaces of the parts,which construction, in cooperation with resilient means urging therespectiveparts of the filler to mutual engagement at the abuttingbeveled ends, provides an essential unbalance of pressure along thelongitudinal adjacent surfaces of the filler and the tube into which itis fitted, thereby assuring presence of the necessary chain of airmolecules to provide a relatively short air gap along the internalsmooth surface of the tube and between conducting electrodes positionedat the respective ends thereof.

Another object is the provision of a novel mechanical reinforcementconstruction associated with the tubular structure which effectivelyreinforces the closed end thereof, and cooperates electrically with thenormally grounded electrode at the vented end of tubular structure toinitiate formation of ionized streamers extending from the vented-endconducting electrode to thereby substantially reduce the sparkovervoltage of the device, and so increase its ability to protect associatedapparatus. Heretofore, devices of this kind in which the tubereinforcement has also functioned as an electrostatic flux controlmember have failed to provide any means for reinforcing the closed endof the tube at which maximum internal pressure is expressed when thedevice operates. Contrary to the conventional arrangements, my inventionherein provides a construction eliminating the necessity for connectingthe reinforcement member to the grounded and vented electrode, andthereby provides a structure having superior strength properties notfound in presently available devices of this character.

With the foregoing and other objects in view which will appear from thefollowing detailed description, my invention resides in the novel form,combination and construction of the components.

The invention is illustrated in the accompanying drawing, the singlefigure of which is a longitudinal sectional view of a device embodyingthe invention.

The particular embodiment shown in the drawing is an excess-voltageprotective device having a discharge chamber formed by a tube or tubularstructure 30 of a suitable insulating material, for example, hard fiber.An upper electrode 16 is threaded into the upper end of the tube 30,here shown as an inverted cup-shaped metal member, and the metal stud 12threadedly extends coaxially downward through the top portion and intothe hollow of the cup-shaped upper electrode 16. In somewhat similarmanner, a lower metal electrode 24 extends upwardly from the bottom endof the tube 30 in threaded engagement therewith, the upper end of thelower electrode 24 being spaced from the lower end of the upperelectrode 16 to form a spark gap for a discharge spark within the tube30. A lower terminal stud 29 extends coaxially upwardly in threadedengagement with the center portion of the lower electrode member 24, andthe vents 25 provide for escape of gas which may be expanded orgenerated within the tube 30. The terminal stud 29 is shielded by meansof the metal sleeve 26, the latter being held in place by the nut 27 ofthe terminal stud 29. An additional nut 28 is provided on the threadedstud 29 for convenience in connecting a ground wire thereto, and for usein mounting the device, if desired.

Within the tubular structure 30 is fitted in tight slipfrictionalengagement with the inner wall thereof a filler of insulating material,such as hard fiber, the filler comprising an upper end portion 19, and alower end portion 20. The lower end of the filler 20 is fitted with ametal ferrule 21, for supporting the filler on the lower electrodemember 24, and a helical spring 17 is centrally positioned between theupper electrode 16 and the upper end of the filler part 19, beingarranged to continuously urge the filler in downward direction againstthe lower electrode 24.

The filler is severed between its respective ends along an oblique planeindicated at the numeral 34 to divide the filler into its two separateparts. The angle of the bevel is preferably small with respect to thelongitudinal axis of the'filler, so that the wedging effected by theresilient endwise urging of the upper portion 19 against the lowerportion 20 will be effective to unbalance the pressure of the respectivefiller parts against the inner surface of the tube 30 alongdiametrically disposed vertical lines, thereby providing the shorterdischarge path from one electrode to the other along the lines of lowestcontact pressure. The compensation for change in dimensions justdescribed is essential, because if the filler istoo tightly fitted, theprotective ability of the device is destroyed, and if too loose,power-follow current will be established in the path of the dischargespark, as in the conventional expulsion type of arrester.

A narrow channel 22, preferably in the form of a furrow generallytriangular in section and having a flat top and an outwardly anddownwardly sloping side, is cut helically in the surface of the fillerfrom its upper to its lower end, thereby providing a relative longdischarge path of relatively less restriction in parallel with theshorter path described above. For best operating results, the channel 22should be of substantially less width than the land 23 between theconvolutions of the channel, and the pitch of the spiral channel shouldbe such that any gas moving therein will move transversely with respectto the shorter discharge path.

To assure proper operation of the device in connection with a 60 cyclealternating current, the R.M.S. voltage should preferably not exceed1800 times the length in inches of the shorter path, and the length ofthe longer spiral path should be at least 2.4 times the length of theshorter path. As a further important consideration, the length of thelands should never be less than the width of the spiral channel, asmeasured vertically along the inner surface of the tube. As anillustration of the important related dimensions of a device meeting therequirements of the principal object as set forth above, a devicesuccessfully operated in connection with a 60 cycle 8000 volt R.M.S.circuit was constructed having the direct shorter path 5 inches inlength, the longer path 12 inches in length, a filler diameter of oneinch, the longer path being formed by a flat-topped furrow ii deep and Awide, the pitch of the spiral defining the longer path 71 and the beveldividing the filler at 30 degrees with the axis of the filler. Thedrawing illustrates such a construction in approximately actual size.

The upper and closed end of the tubular structure 30 is reinforced bysnugly fitting thereover a cup-shaped steel reinforcing member 15, thelatter extending downwardly for such a distance that the spacing betweenthe upper electrode 16 and the lower electrode 24 is greater than thespacing between the latter and the lower end of the reinforcing member15. To assure that sparkover between the electrodes 24 and 16 will occurinside of the spark chamber 30, the outer surface of the lower endportion of the latter is uniformly tapered, and thereover is tightlyfitted a plastic sleeve 31 of an insulating material such as adielectric grade of polyethylene. The sleeve 31 is held firmly underpressure by its contact at its lower end surface with the outwardlyextending flange portion of the lower electrode 24, and is extendedupwardly beyond the lower end of the tube reinforcing member 16 andthereover to an elevation near the upper end of the tube 30. As anadditional means of increasing the insulation between the reinforcingmember 15 and the lower electrode 24, a silicone grease or a thin coatof dielectric adhesive (not shown) may be applied between the plasticsleeve 31 and the lower end of the tube 30.

A housing 32, of porcelain or similar material, is pro.- vided to housethe elements of the discharge chamber,

- being supported at its lower end by the outwardly extended flange ofthe lower electrode 24, and positioned and covered at its upper end bythe metal cover 14, being held in position by the nut 13 of the stud 12,the latter extending through a centrally-positioned opening in the cover14. Gaskets 18 and 33 may be provided to seal the points at therespective ends of the housing 32.

In operation, the upper terminal 12 is connected, through an externalseries spark gap 11, to a terminal of a protected device or to aconductor therewith associated, the lower terminal 29 being connected toground. Upon the occurrence of a predetermined excess voltage, such asmight be caused by lightning, the gap 11 and the internal shorter gapbetween the electrodes 16 and 24 sparks over along the internal surfaceof the tube 30, discharging the surge to ground. The resistance of thepair of discharge paths is maintained at such a high value immediatelyfollowing the discharge spark that the power line voltage is unable tomaintain continued flow of power-follow current across the gap 11, andno followcurrent occurs regardless of the polarity or instantaneousvoltage of the connected conductor at the time of discharge.

While I am not certain as to the correct explanation for the improvedresults I obtain, it seems likely that the heat from the discharge orlightning spark causes a sudden transverse movement of air and perhapssome water vapor from the fiber members so that the discharge spark isso moved, cooled and elongated by the movement of expanded gases as toprevent the necessary reduction in arcarresting resistance to enable afollow-current arc to be kindled. Since there is no apparent thermaldegradation of the fiber members, no large volume of decomposition gasesare evolved, such as is usual in conventional types of devices. Theabsence of the evolution of large quantities of incandescent gases maybe noted by observing the device under operating conditions. The initialdischarge spark follows the highly but least restricted path asdetermined by the relation of the filler parts to the inner surface ofthe tube as established by the wedging action, so that any movement ofthe discharge spark as it may be driven by the moving gas is necessarilyinto an even more restricted or perhaps fully blocked path, which ofcourse has a resistance approaching the puncture strength of theinsulating material.

It will be apparent from the foregoing description that modificationsand variations of the present invention are possible. It is, therefore,to be understood that the invention is not limited in its application toall of the details of construction and arrangement of the partsspecifically described or illustrated, and that, within the scope of theappended claims, it may be practiced otherwise than as specificallydescribed or illustrated.

I claim:

1. An excess-voltage protective device subject to follow-current inresponse to a precursory discharge spark therein, comprising a hollowinsulator body forming a housing for a plurality of normally insulatedconducting electrodes positioned in spaced relationship to provide anair gap, and a dielectric structure formed of insulating material andconfining said gap, said dielectric structure including a tubularstructure vented at one end, said tubular structure having a smoothinner surface, and an elongated filler of insulating material tightlyfitted therein between said electrodes, said filler member comprising atleast two parts disposed in beveled end-to-end relationship and having aconvoluted surface including a furrow extending helically around thefiller member, the width of the land between adjacent convolutions ofsaid furrow being at least equal to the width of the furrow, saidconvoluted surface cooperating with said smooth surface to define one ofa pair of separate paths bridging the con fined air gap, the other ofsaid pair extending along said inner surface and forming a shorter path,both of said paths ofiering a composite arc arresting resistance ofsufiicient magnitude to preclude the formation of any follow-current arewithin the device following sparkover of said shorter path.

2. An excess-voltage protective device comprising a tubular structure ofinsulating material, spaced electrode members disposed at the respectiveends of said tubular structure, at least one of said electrode membersextending into said tubular structure and at least one end of saidtubular structure being vented, and an elongated filler member ofinsulating material fitted within said tubular structure between saidelectrode members, said filler member having a furrow extendinghelically from one electrode member to the other electrode member, thewidth of the land between adjacent convolutions of said furrow beingsubstantially greater than the width of said furrow.

3. An excess-voltage protective device comprising a tubular structure ofinsulating material, spaced electrode members disposed at the respectiveends of said tubular structure, at least one of said electrode membersextending into said tubular structure and at least one end of saidtubular structure being vented, and a segmented filler member snuglyfitted within said tubular structure between said electrode members,said filler member having a furrow extending helically from oneelectrode member to the other electrode member, the width of the landbetween adjacent convolutions of said furrow being at least equal to thewidth of said furrow, and the segments of said filler abutting inbeveled-end engagement.

4. An excess-voltage protective device comprising a tubular structure ofinsulating material, spaced electrode members disposed at the respectiveends of said tubular structure, at least one of said electrode membersextending into said tubular structure and at least one end of saidtubular structure being vented, a segmented filler member tightly fittedwithin said tubular structure between said electrode members, saidfiller member having a furrow extending helically from one electrodemember to the other electrode member, the width of the land betweenadjacent convolutions of said furrow being substantially greater thanthe width of said furrow, the segments of said filler extending inend-to-end relationship and engaging one another along matchingbeveled-end surfaces, and resilient means urging said segments toengagement.

5. An excess-voltage protective device comprising a tubular insulatingstructure, normally insulated conducting electrodes disposed at therespective ends of said tubular structure, at least one of saidelectrodes extending partway into said tubular structure to assure thatsparkover between said electrodes results in a discharge spark withinsaid tubular structure, an elongated filler tightly fitted into saidtubular structure and positioned between said electrodes, and structuralmeans for assuring that the voltage required to cause a discharge sparkbetween said electrodes is not substantially increased due to thepresence of said tightly-fitted filler therebetween, said meansincluding the division between said electrodes of said filler into twoparts along an oblique plane, and resilient means longitudinally urgingsaid parts in abutting end-toend engagement.

References Cited in the file of this patent UNITED STATES PATENTS2,591,370 Nelson et al. Apr. 1, 1952 2,673,941 De Val Mar. 30, 19542,677,072 De Val Apr. 27, 1954 2,683,235 Roloson July 6, 1954 2,691,742De Val Oct. 12, 1954

